Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15A0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007, 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
Lithium/sulfur (Li/S) batteries are particularly attractive because of the balance between high specific energy (i.e., >350 Wh/kg has been demonstrated), rate capability, and cycle life (>50 cycles). Only lithium/air batteries have a higher theoretical specific energy. Lithium/air batteries, however, have very limited rechargeability and are still considered primary batteries.
Thus the advantage of using a Li metal anode is the much higher energy density of the entire cell, as compared to cells with graphitic or other intercalation anodes. A disadvantage of using pure Li metal is that lithium is highly reactive. Accordingly, the lithium metal can be damaged by other chemical species in the cell. Additionally, the lithium metal has a propensity to grow metallic dendrites when the cell is being charged. Metallic dendrites can then puncture the separator and cause an internal short of the cell.
Moreover, repeated cycling of a Li-anode cell results in significant morphology changes. The initially dense metal, after a certain number of cycles, develops surface roughness and a sponge-like morphology. This morphology is potentially dangerous due to high surface area which increases the chance for, and severity of, runaway reactions.
What is needed therefore is a battery system and method that reduces the potential for dendrite formation and undesired morphological changes in the anode. A system and method which could also be used to provide more accurate state of charge determination in a cell would be beneficial.